Revising purely off of cgp revision guide Watch
For me, I wrote detailed notes for biology and chemistry, and I managed to do decently (got 9s for both of them).
For physics, I didn't have time to write notes; I hadn't written any notes for the subject at all over the two year course & the exam was close to my music and english exams, which I cared a lot about. Therefore, I only had the CGP to get me through the exam and I was very, very worried. Turns out that it worked - I got a 9 too, and funny enough, I had the highest raw mark for physics out of chem & bio! Proves that the CGP is good enough; don't bother writing detailed notes like I did.. Waste of time that could've been better spent doing past paper questions imo.
It's not that there's a maximum grade if you study solely from the revision guides, but for mathematical and scientific subjects especially I would advise you practice using past papers as well so that you can gain familiarity with doing questions yourself rather than just looking at worked examples. It's easy to look at something and think that you understand it - it's an entirely different kettle of fish to then apply that information to an unfamiliar context
I wouldn’t say there’s a ‘top grade’ you can get by using revision guides, but I would say that for e.g. an A* you’d need to know close to 100% of the revision guide, whereas 70-80% of a textbook for the same grade. It really depends how you learn best, how much time you have, how interested you are in Science and whether you intend to study them further. As others have suggested though, I’d recommend doing some past paper questions in addition to using a revision guide/textbook. Good luck with your exams!